1. Field of the Invention
The present invention relates to electrically controlled hydraulic systems for operating machinery, and in particular to determining in which one of a plurality of hydraulic fluid metering modes the system should operate at any given time.
2. Description of the Related Art
A wide variety of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid operated valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates sophisticated computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a proportional solenoid operated spool valve that is well known for controlling the flow of hydraulic fluid. Such a valve employs an electromagnetic coil which moves an armature connected to the spool that controls the flow of fluid through the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or the spool is spring loaded to close the valve when electric current is removed from the solenoid coil. Alternatively a second electromagnetic coil and armature is provided to move the spool in the opposite direction.
When an operator desires to move a member on the machine a joystick is operated to produce an electrical signal indicative of the direction and desired rate at which the corresponding hydraulic actuator is to move. The faster the actuator is desired to move the farther the joystick is moved from its neutral position. A control circuit receives a joystick signal and responds by producing a signal to open the associated valve. A solenoid moves the spool valve to supply pressurized fluid through an inlet orifice to the cylinder chamber on one side of the piston and to allow fluid being forced from the opposite cylinder chamber to drain through an outlet orifice to a reservoir, or tank. A hydromechanical pressure compensator maintains a nominal pressure (margin) across the inlet orifice portion of the spool valve. By varying the degree to which the inlet orifice is opened (i.e. by changing its valve coefficient), the rate of flow into the cylinder chamber can be varied, thereby moving the piston at proportionally different speeds. A given amount of electric current applied to the valve's solenoid achieves the desired inlet orifice valve coefficient. Thus prior control algorithms were based primarily on inlet orifice metering using an external hydromechanical pressure compensator.
Recently a set of proportional solenoid operated pilot valves has been developed to control fluid flow to and from the chambers of a cylinder, as described in U.S. Pat. No. 5,878,647. One pair of valves controls the flow of fluid from a supply line into the cylinder chambers and the another pair of valves controls the flow of fluid from the cylinder chambers into a tank return line. By selectively opening the proper valve in each pair, the cylinder can extend or retract its piston. These modes of metering fluid to and from the cylinder are referred to as “powered extension” and “powered retraction.”
Hydraulic systems also employ regeneration modes of operation in which fluid being drained from one cylinder chamber is fed back through the valve assembly to supply the other cylinder chamber. The pair of valves connected to the supply line may be opened to connect the cylinder chambers in the “high side regeneration” metering mode or the pair of valves connected to the return line may be opened to connect the cylinder chambers in the “low side regeneration” metering mode. Heretofore, the mode of operation typically was selected manually by the machine operator. However, it is desirable to provide automatic mode selection.